1. Field of the Invention
The invention relates to a method for detecting impermissibly high scaling of an optical measurement section for measuring the turbidity of a washing liquid in a domestic appliance that is operated with hard water, such as a washing machine or dishwasher. The appliance has an evaluation circuit for the measured signals from the measurement section and a microprocessor for converting the measurement results into the sequence control system of the appliance.
In the case of appliances to which hard water is supplied, such as washing machines or dishwashers, there is the problem that the service life of the appliance or else the performance of the appliance may be adversely affected as a result of chalk deposits. It is often not possible for such chalk deposits to be detected by the user of the appliance, or the degree to which these deposits are damaging cannot be assessed by the user.
German published, non-prosecuted application 195 21 326 A1 (corresponding U.S. application Ser. No. 08/877,101; commonly owned) discloses a method for calibrating turbidity measured values, in which the measured values are calibrated as a function of variable temperatures. However, that process does not take another variable influence, namely the risk of scaling of the optical measurement section, into consideration. Since this measurement section dries out again after each working process--this is because, for reasons not explained specifically here, it is best located far above the level of the amount of residual water that remains after the washing liquid has been pumped out of the container of the machine--it is possible for residual water droplets to leave chalky edges behind when they dry out, and these gradually grow to form a dense covering. At first, slight chalk deposits are barely disruptive. However, when a covering that is also visible to the naked eye has been produced, then, even in the case of clear water, the measurement beam in the optical measurement section will already be attenuated to such an extent that the signal evaluation circuit will detect a supposed washing-liquid turbidity which depends on the thickness of the covering.